This invention relates generally to radial lip seals for use with relatively rotatable members, and more particularly to a method of making a composite lip seal formed of two dissimilar materials.
Lip-type fluid seals typically employ a flexible sealing member that has one or more sealing lips adapted to engage against a sealing surface of a relatively rotatable member. The purpose of a lip-type fluid seal is to retain application fluids, usually lubricants, and to prevent ingress of environmental contaminants. The selection of the sealing member material is determined by a number of factors, including the environment in which the seal is operating. Elastomerics are often employed as the sealing lip material in applications where the lip seal is exposed to a dirty environment containing such contaminants as water, salt, and abrasives such as dirt, mud, sand, etc. which would be harmful to axle bearings used in certain heavy equipment applications, for example. The lip seal may also be made of other materials, such as polytetrafluoroethylene (PTFE) materials. However, elastomers are far more resistant to wear than PTFE materials under the contaminated conditions referred to above. On the other hand, certain environmental conditions favor PTFE materials over elastomers, such conditions including high and low temperature and/or dry or low lubricant running conditions-.
It is not uncommon for the seal to be subjected not only to a contaminated environment, but also to low or high temperature and/or low lubricity conditions. In the past, a sealing lip with a composite elastomeric/PTFE lip edge has been employed, an example of which is disclosed in U.S. Pat. No. 6,213,476 assigned to the assignee of this invention. However difficulties have sometimes been encountered in the manufacture of such composite elastomeric/PTFE sealing lips.
It has been found that in molding elastomeric material to an insert of PTFE to form a composite elastomeric/PTFE sealing lip, the elastomeric material, as it flows into the mold, tends to displace or shift the PTFE insert in a condition called xe2x80x9crunning.xe2x80x9d The fact that the PTFE insert is typically relatively thin, usually having a thickness of 0.020 inches or less, makes it even more difficult to hold the insert stationary while bonding to the elastomeric material in a compression mold. In accordance with the present invention, it has been discovered that by forming an enlargement on one end of the PTFE insert, the insert is more easily handled when loading it into the mold cavity and will remain in place during the molding process.
Further in accordance with the invention, a wafer of PTFE material having a body portion and an enlarged end portion of greater thickness than the body portion, is supported in the mold cavity. Elastomeric material is forced into the mold cavity to form a molded preform of the elastomeric sealing member and bond it to the wafer. After the preform and bonded wafer are removed from the mold cavity, an end portion of the preform and the enlarged end portion of the wafer are trimmed to form the elastomeric member and the PTFE insert. To further assist in holding the wafer during the molding process, the mold is preferably designed with a built-in lobe engagable with a base end of the wafer.